Low hub-to-tip ratio fan for a turbofan gas turbine engine

ABSTRACT

A fan for a turbofan gas turbine engine having a low hub-to-tip ratio is disclosed. The fan includes a rotor hub and a plurality of radially extending fan blades. Each fan blade defines a hub radius (R HUB ), which is the radius of the leading edge at the hub relative to a centerline of the fan, and a tip radius (R TIP ), which is the radius of the leading edge at a tip of the fan blade relative to the centerline of the fan. The ratio of the hub radius to the tip radius (R HUB /R TIP ) is less than 0.29. In a particular embodiment, this ratio is between 0.25 and 0.29. In another particular embodiment, this ratio is less than 0.25.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 13/687,540filed Nov. 28, 2012, the entire content of which is incorporated hereinby reference.

TECHNICAL FIELD

The present invention relates to turbofan engines and more particularlyto fans for such engines having low hub to tip ratios.

BACKGROUND

Most gas turbine engine fans are composed of a central hub onto which aplurality of separately formed fan blades are secured. Integrated bladedrotor (IBR) fans are known for their relative lightness and thereforeare desirable, however known IBR fans cannot be formed having a low hubto tip radius ratio because of limitations in manufacturingcapabilities. Such a low hub to tip radius ratio is however desirablebecause it means the maximum diameter of the fan can be reduced withoutnegatively effecting performance. Reducing the overall diameter of thefan reduces weight and improves the efficiency of the fan.

Therefore, while the advantages of reducing the ratio of the radius ofthe hub to the radius of the tip are well appreciated in terms ofreducing the specific flow of air entering the leading edge of the fan,attempts to date to reduce the specific flow by reducing this ratio havenot been readily possible, particularly for IBR fans. Attempts tomanufacture an integrated bladed rotor (IBR) fan with a low hub to tipratio have not been successful because of the lack of space for machinetools between the roots of the blades when the hub is also reduced insize.

SUMMARY

There is accordingly provided a fan for a turbofan gas turbine engine,the fan defining a fan centerline and comprising a rotor hub and aplurality of fan blades adapted to rotate about the fan centerline, thefan blades extending radially from the rotor hub to outer tips thereof,the fan blades circumferentially spaced apart about the rotor hub in asingle axial blade row, each of the fan blades having a leading edge, ahub radius (RHUB) and a tip radius (RTIP), wherein the hub radius (RHUB)is the radius of the leading edge at the hub relative to the fancenterline, and the tip radius (RTIP) is the radius of the leading edgeat the outer tip relative to the fan centerline, and wherein the ratioof the hub radius to the tip radius (RHUB/RTIP) is less than 0.29.

The ratio of the hub radius to the tip radius (RHUB/RTIP) of the fan asdescribed above may be more particularly is less than 0.25.

The ratio of the hub radius to the tip radius (RHUB/RTIP) of the fan asdescribed above may be more particularly between 0.25 and 0.29.

The fan as described above may be an integrally bladed rotor, the fanblades being integrally formed with the rotor hub.

The rotor hub of the integrally bladed rotor of the fan as describedabove may include a preform having root stubs disposed on the rotor hubat circumferential positions corresponding to at least alternate ones ofsaid fan blades in the single axial blade row, the root stubs beingformed on the rotor hub prior to the fan blades being fastened thereto.

The root stubs of the hub of the integrally bladed rotor of the fan asdescribed above may have airfoils welded thereto to form the fan bladesof the integrally bladed rotor.

The airfoils of the integrally bladed rotor of the fan as describedabove may be linear-friction-welded to the respective root stubs.

The preform of the integrally bladed rotor of the fan as described abovemay have the root stubs disposed on the rotor hub for each of the fanblades in the single blade row.

There is also provided a turbofan gas turbine engine including a fanupstream of at least one compressor and defining a fan centerline aboutwhich the fan rotates, the fan comprising: a rotor hub and a pluralityof fan blades substantially radially extending from the rotor hub toouter tips thereof, the fan blades being arranged in a single blade rowon the rotor hub, each of the fan blades of the single blade row havingan airfoil with a leading edge, the leading edge of the airfoilextending from a hub radius (RHUB) at the rotor hub to a tip radius(RTIP) at the outer tip, and wherein a ratio of the hub radius to thetip radius (RHUB/RTIP) is less than 0.29.

The ratio of the hub radius to the tip radius (RHUB/RTIP) of theturbofan gas turbine engine as described above may be between 0.25 and0.29.

The ratio of the hub radius to the tip radius (RHUB/RTIP) of theturbofan gas turbine engine as described above may be less than 0.25.

The fan of the turbofan gas turbine engine as described above may be anintegrally bladed rotor, the airfoils of the fan blades being integrallyformed with the rotor hub.

The rotor hub of the integrally bladed rotor as described above mayinclude a preform having root stubs disposed on the rotor hub atcircumferential positions corresponding to at least alternate ones ofsaid fan blades in the single axial blade row, the root stubs beingformed on the rotor hub prior to the airfoils being fastened thereto.

The airfoils of the integrally bladed rotor as described above may bewelded to the root stubs to form the fan blades of the integrally bladedrotor.

The airfoils of the integrally bladed rotor as described above may belinear-friction-welded to the respective root stubs.

The preform of the integrally bladed rotor as described above may havethe root stubs disposed on the rotor hub for each of the fan blades inthe single blade row.

There is further provided a method of manufacturing a fan for a turbofangas turbine engine, the fan adapted to rotate about a fan centerlineaxis, the method comprising: providing a rotor hub having an outerperipheral surface defining a hub radius relative to the fan centerlineaxis; selecting a predetermined length of fan blade airfoils, thepredetermined length selected such that a ratio of the hub radius to atip radius of the blade airfoils, as measured from the fan centerlineaxis to tips of the fan blade airfoils once the fan blade airfoils aremounted to the rotor hub, is less than 0.29; and positioning said fanblade airfoils to the outer peripheral surface of the rotor hub in anaxially aligned single blade row, the fan blade airfoils beingcircumferentially spaced about the rotor hub within the axially alignedsingle blade row.

The method as described above may further comprise integrally formingthe fan blade airfoils and the rotor hub to produce an integrally bladedrotor.

The method as described above may further comprise forming a rotor hubpreform having a number of root stubs circumferentially spaced apart ona periphery of the rotor hub, the root stubs being axially aligned todefine said single blade row, and fastening the fan blade airfoils tothe root stubs to form fan blades integrally formed with the rotor hub.

The step of selecting of the method as described above may furthercomprise selecting the predetermined length of the fan blade airfoilssuch that the ratio of the hub radius to the tip radius of the fan bladeairfoils is between 0.25 and 0.29.

The step of selecting of the method as described above may furthercomprise selecting the predetermined length of the fan blade airfoilssuch that the ratio of the hub radius to the tip radius of the fan bladeairfoils is less than 0.25.

The method as described above may further comprise integrally formingcircumferentially alternate ones of said fan blade airfoils with the hubpreform directly to the outer peripheral surface of the rotor hubpreform without root stubs, leaving alternate root stubs on the hubpreform to provide access for machine tools between thecircumferentially alternate ones of said fan blade airfoils.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a schematic cross-sectional view of a gas turbine enginehaving a fan in accordance with the present disclosure; and

FIG. 2 is a partial axial cross-sectional view of an embodiment of thefan of the present disclosure.

DETAILED DESCRIPTION

FIG. 1 illustrates a turbofan gas turbine engine 10 generally comprisingin serial flow communication, a fan assembly 12 through which ambientair is propelled, and a core including a compressor section 14 forpressurizing the air, a combustor 16 in which the compressed air ismixed with fuel and ignited for generating an annular stream of hotcombustion gases, and a turbine section 18 for extracting energy fromthe combustion gases. A centerline main engine axis 13 extendslongitudinally through the turbofan engine 10.

The fan 12 propels air through both the engine core and the bypass duct22, and may be mounted to the low pressure main engine shaft 11. The fan12 includes a plurality of radially extending fan blades 20 and acentral hub as will be seen, which hub has a nose cone 22 mountedthereto to protect the hub. As will be described in greater detailbelow, the fan 12 may be an integrally bladed rotor (IBR), in which casethe fan blades 20 are integrally formed with the central hub that isfastened to the low pressure (LP) engine shaft 11 for rotationtherewith.

Referring now to FIG. 2, the fan 12 comprises a plurality of fan blades20 integrally formed with, and substantially radially extending from, acentral fan hub 36 which is mounted to an engine shaft, such as the lowpressure shaft 11, by means of one or more hub support portions 38 whichare also integrally formed with the hub. Each of the blades 20 definesan airfoil 28 which has a leading edge 34 which extends from a bladeroot 30 to a blade tip 40. The blade 20 is integrated with the hub 36,i.e. such the blades 20 are integrally formed as a monolithic componentwith the fan hub 36 to form an IBR fan. The nose cone 22 of the enginemay be fastened to an upstream end of the fan hub 36 by a plurality offasteners 29.

When the radius of the leading edge 30 on the hub 36 is reduced whilethe radius of the blade tip at 40 is maintained, the flow area (FA) ofthe fan 20 is increased thus reducing the specific flow (SF). As seen inFIG. 2, the gaspath through the fan 12 is defined by the annular areabetween the hubs 30 and the tips 40 of the fan blades 20. The radius ofthe fan hub (R_(HUB)), measured at the leading edge 34 of the blade 20,defines the radially inner gaspath boundary and the radius of the bladetip (R_(TIP)), also measured at the leading edge 34, defines theradially outer gaspath boundary. The specific flow of the fan 12 istherefore defined as the mass flow (MF) of air entering the leading edgeof the fan 12, divided by the flow area (FA) at the fan leading edge,normal to the engine axis 13.

The hub to tip ratio of the fan 12 is defined as the ratio of the radiusof the fan hub (R_(HUB)) at the leading edge divided by the radius offan blade tip (R_(TIP)) at the leading edge. As shown in FIG. 2, theseradii are measured from the engine centerline axis 13.

Thus, specific flow is determined as follows:SF=MF/FA,where SF is the specific flow, MF is the mass flow, and FA the flowarea. Reduction of this SF of the fan is desirable as a reduced SF helpsto improve the overall aerodynamic efficiency of the fan because of thelower air velocity.

A reduction in the hub to tip ratio (R_(HUB)/R_(TIP)) will thereforealso cause a reduction in the specific flow (SF) of the fan.Alternatively, the radius of both the hub 36 and the blade tip 40 can bereduced while retaining the same specific flow SF. However, the ratio ofthe hub to tip radii is preferably reduced. Accordingly, the present IBRfan 12 has a ratio of the hub radius to the tip radius, i.e.R_(HUB)/R_(TIP), which is at least less than 0.29. In a particularembodiment, the ratio of the hub radius to the tip radius(R_(HUB)/R_(TIP)) is between about 0.25 and about 0.29. In a furtherparticular embodiment, the ratio of the hub radius to the tip radius(R_(HUB)/R_(TIP)) is less than or equal to 0.25.

The advantage of a lower tip radius is a smaller diameter fan andtherefore a lighter weight engine. Lowering the hub leading edge radiusalso changes the flow angle of the airstream, and the resulting rearwardsweep in the lower portion of the fan blade airfoils 28 improvesperformance by reducing the leading edge velocities through the sweepeffect and also draws flow towards the hub 36 which helps to reduce flowseparation that the blade root.

The advantage of using the integrally bladed rotor (IBR) fan 12 is itsreduced weight compared to a traditional detachable bladed rotor. Themachining of an IBR fan 12 with such a low hub/tip ratio is madedifficult by the lack of space between the blades 20, particularly atthe blade roots 30 since the gap between the blades is much narrower thesmaller the radius of the fan.

However, in one particular method of manufacturing the IBR fan 12described herein, it has been found that by first machining a root stub44 on the hub 36, or more specifically on a hub preform, the lower hubradius, and more particularly the low hub to tip radius ratios describedabove, can be obtained because it is easier to access the radial gapbetween adjacent blades 20 with machine tools. The blade airfoils 28 maythen be fixed to the root stubs 44 of the hub preform be welded byLinear Friction Welding (LFW), for example, along the joint line 42 asshown on the blade 20 in FIG. 2. It has been contemplated thatalternative methods may also be used, such as forming a root stub 44only for every alternate blade, while machining the full blade 20between each alternate root stub. This would allow sufficient access formachine tools between two alternate full blades, to machine around thearound the remaining root stub.

Thus, a low-weight fan 12 as described herein is achieve, because of itsintegrated bladed rotor construction, and which provides a hub to tipradius ratio of at least less than 0.29, and more particularly between0.25 and 0.29, and more particularly still a hub to tip radius ratio of0.25 or less.

The above description is meant to be exemplary only, and one skilled inthe art will recognize that changes may be made to the embodimentsdescribed while still falling within the scope of the appended claims,which define the present invention. Such modifications will be apparentto those skilled in the art, in light of a review of this disclosure.

The invention claimed is:
 1. A fan for a turbofan gas turbine engine,the fan defining a fan centerline and comprising a rotor hub and aplurality of fan blades adapted to rotate about the fan centerline, thefan blades extending radially from the rotor hub to outer tips thereof,the fan blades circumferentially spaced apart about the rotor hub in asingle axial blade row, each of the fan blades having a leading edge, ahub radius (R_(HUB)) and a tip radius (R_(TIP)), wherein the hub radius(R_(HUB)) is the radius of the leading edge at the hub relative to thefan centerline, and the tip radius (R_(TIP)) is the radius of theleading edge at the outer tip relative to the fan centerline, andwherein the ratio of the hub radius to the tip radius (R_(HUB)/R_(TIP))is less than 0.29.
 2. The fan as defined in claim 1, wherein the ratioof the hub radius to the tip radius (R_(HUB)/R_(TIP)) is less than 0.25.3. The fan as defined in claim 1, wherein the ratio of the hub radius tothe tip radius (R_(HUB)/R_(TIP)) is between 0.25 and 0.29.
 4. The fan asdefined in claim 1, wherein the fan is an integrally bladed rotor, thefan blades being integrally formed with the rotor hub.
 5. The fan asdefined in claim 4, wherein the rotor hub of the integrally bladed rotorincludes a preform having root stubs disposed on the rotor hub atcircumferential positions corresponding to at least alternate ones ofsaid fan blades in the single axial blade row, the root stubs beingformed on the rotor hub prior to the fan blades being fastened thereto.6. The fan as defined in claim 5, wherein the root stubs have airfoilswelded thereto to form the fan blades of the integrally bladed rotor. 7.The fan as defined in claim 6, wherein the airfoils arelinear-friction-welded to the respective root stubs.
 8. The fan asdefined in claim 5, wherein the preform has said root stubs disposed onthe rotor hub for each of the fan blades in the single blade row.
 9. Aturbofan gas turbine engine including a fan upstream of at least onecompressor and defining a fan centerline about which the fan rotates,the fan comprising: a rotor hub and a plurality of fan bladessubstantially radially extending from the rotor hub to outer tipsthereof, the fan blades being arranged in a single blade row on therotor hub, each of the fan blades of the single blade row having anairfoil with a leading edge, the leading edge of the airfoil extendingfrom a hub radius (R_(HUB)) at the rotor hub to a tip radius (R_(TIP))at the outer tip, and wherein a ratio of the hub radius to the tipradius (R_(HUB)/R_(TIP)) is less than 0.29.
 10. The turbofan gas turbineengine as defined in claim 9, wherein the ratio of the hub radius to thetip radius (R_(HUB)/R_(TIP)) is between 0.25 and 0.29.
 11. The turbofangas turbine engine as defined in claim 9, wherein the ratio of the hubradius to the tip radius (R_(HUB)/R_(TIP)) is less than 0.25.
 12. Theturbofan gas turbine engine as defined in claim 9, wherein the fan is anintegrally bladed rotor, the airfoils of the fan blades being integrallyformed with the rotor hub.
 13. The turbofan gas turbine engine asdefined in claim 12, wherein the rotor hub of the integrally bladedrotor includes a preform having root stubs disposed on the rotor hub atcircumferential positions corresponding to at least alternate ones ofsaid fan blades in the single axial blade row, the root stubs beingformed on the rotor hub prior to the airfoils being fastened thereto.14. The turbofan gas turbine engine as defined in claim 13, wherein theairfoils are welded to the root stubs to form the fan blades of theintegrally bladed rotor.
 15. The turbofan gas turbine engine as definedin claim 14, wherein the airfoils are linear-friction-welded to therespective root stubs.
 16. The turbofan gas turbine engine as defined inclaim 13, wherein the preform has said root stubs disposed on the rotorhub for each of the fan blades in the single blade row.
 17. A method ofmanufacturing a fan for a turbofan gas turbine engine, the fan adaptedto rotate about a fan centerline axis, the method comprising: providinga rotor hub having an outer peripheral surface defining a hub radiusrelative to the fan centerline axis; selecting a predetermined length offan blade airfoils, the predetermined length selected such that a ratioof the hub radius to a tip radius of the blade airfoils, as measuredfrom the fan centerline axis to tips of the fan blade airfoils once thefan blade airfoils are mounted to the rotor hub, is less than 0.29; andpositioning said fan blade airfoils to the outer peripheral surface ofthe rotor hub in an axially aligned single blade row, the fan bladeairfoils being circumferentially spaced about the rotor hub within theaxially aligned single blade row.
 18. The method of claim 17, furthercomprising integrally forming the fan blade airfoils and the rotor hubto produce an integrally bladed rotor.
 19. The method of claim 18,further comprising forming a rotor hub preform having a number of rootstubs circumferentially spaced apart on a periphery of the rotor hub,the root stubs being axially aligned to define said single blade row,and fastening the fan blade airfoils to the root stubs to form fanblades integrally formed with the rotor hub.
 20. The method as definedin claim 16, wherein the step of selecting further comprises selectingthe predetermined length of the fan blade airfoils such that the ratioof the hub radius to the tip radius of the fan blade airfoils is between0.25 and 0.29.
 21. The method as defined in claim 16, wherein the stepof selecting further comprises selecting the predetermined length of thefan blade airfoils such that the ratio of the hub radius to the tipradius of the fan blade airfoils is less than 0.25.
 22. The method asdefined in claim 19, further comprising integrally formingcircumferentially alternate ones of said fan blade airfoils with the hubpreform directly to the outer peripheral surface of the rotor hubpreform without root stubs, leaving alternate root stubs on the hubpreform to provide access for machine tools between thecircumferentially alternate ones of said fan blade airfoils.